RESPIRATION 



663 



the nitrogen is merely diffused into the blood, the 

 oxygen and the carbonic acid must be combined with 

 some substance or substances in the blood. If we 

 gradually lower the external pressure of the atmo- 

 sphere upon the blood we notice that at any given 

 temperature (at which the combination can exist) 

 the pressure may be lowered to a certain point 

 without much gas coming off, and that at that 

 point the gases begin to come off rapidly. This is 

 another proof that the gases are combined and not 

 merely absorl>ed in the blood ; for in case of simple 

 absorption the gases come off in equal amounts for 

 equal lowering? of pressure. The amount of the 

 gases that can be taken from blood-plasma (free 

 frcim blood-cells) is 0'26 vols. of oxygen, 35 '26 vols. 

 of carltonic acid, and 2-24 vols. of nitrogen. The 

 great mass of the oxygen is, therefore, not in the 

 plasma, but in the corpuscles ; while the great mass 

 of the carlwnic acid is in the plasma. The oxygen 

 is found to be united to the red colouring matter, 

 of which the red blood-cells are chiefly composed. 

 This substance is called haemoglobin. It is not 

 MI easy to determine in what combination the 

 carl>onic acid exists in the plasma. A certain 

 amount is found in the red corpuscles ( though the 

 above figures do not show it ) ; indeed, some writers 

 consider that the haemoglobin of these cells is the 

 chief carrier of carbonic acid. The effect of lowered 

 pressure upon blood-plasma, so far us regards 

 carbonic acid, is much the same as it is upon solu- 

 tions of sodium hydrogen carbonate. Some writers 

 believe that the carlionic acid exists in the plasma 

 in the form of sodium bicarlonate. Others oelieve 

 that it may be in the form of bisodium hydrogen 

 phosphate. The presence of red blood-corpuscles 

 has a very marked effect upon the disengagement 

 of carbonic acid under lowered pressure ; it hastens 

 it considerably. This effect appears to be due to 

 the presence of oxyhcemnglobin. 



The total pressure of the atmosphere is 760 

 mm. of mercury. The partial pressure of oxygen 

 in the air is 159'6 ; of carbonic nciit, practically zero ; 

 of nitrogen, 600"4. Oxygen does not leave arterial 

 blood until the partial pressure falls to 29'64, nor 

 venous blood until the pressure falls to 22'04 ; 

 these therefore are the partial pressures of oxygen 

 in arterial and venous blood. Carbonic acid does not 

 leave arterial Mood until the partial pressure falls 

 to 21-18, and venous blood until it falls to 41-04. 

 Therefore blood exposed to air would readily gain 

 oxygen and lose carlionic acid. But the air in the 

 part of the lungs where the respiratory interchange 

 takes place is not the same as the air Bur-rounding 

 the body ; the partial pressures of expired air will 

 be nearer the true numbers ; they are of oxygen, 

 121-6 ; of carbonic acid, 33'4 ; of nitrogen, 600. But 

 even expired air is not the same as air within the 

 alveoli; for the air taken in and out of the lungs 

 ( tidal air) only enters and leaves the larger respira- 

 tory passages near the opening into the outer air ; 

 from these it diffuses into the air of the alveoli. 

 The partial pressures of this air have been esti- 

 mated by introducing a collector into the alveoli and 

 taking out samples. Specimens of air collected in 

 this way have been found to have the following 

 partial pressures: Oxygen, 27"44 ; carlnmic acid^ 

 27-06; nitrogen, 705-5. It is difficult to believe 

 that this is a correct estimate, for the difference be- 

 tween the partial pressure in the alveoli and that in 

 the expired air is so enormous. However, assum- 

 ing it to be correct, the following diagram will show 

 the direction in which diffusion must take place. 



Venom* Blood. Alveolar Air. 

 Oxygen 2204 27'44 



Carbonic Acid 41-04 21'04 



The vertical line represents the alveolar and 



capillary wall ; the arrows show the direction in 

 which the gas molecules must diffuse. But if we 

 compare the partial pressures in venous blood, in 

 arterial blood, and in alveolar air, a very remark- 

 able fact appears. 



Alveolar Air. 



Oxygen 27 '44 



Carbonic AciA.27-06 



Venous 

 Blood. 



22-04 

 41-04 



Arterial Blood. 

 29-64 

 21-04 



The venous blood flows through the lungs, and 

 issues as arterial blood, and yet the partial pressure 

 of oxygen in arterial blood is higher than it is in 

 alveolar air, the place from which it must have 

 come ; while the pressure of carbonic acid in 

 arterial blood is Imver than it is in alveolar air, the 

 place to which it has passed. We must therefore 

 conclude that the living alveolar wall has exercised 

 some influence upon the gases in virtue of its secret- 

 ing and excreting activity ; it has done work 

 against the molecular energies that produce diffu- 

 sion. But the numbers given by various authors 

 for the partial pressures of the gases in the various 

 places differ, so that perhaps no thoroughly reliable 

 conclusion can l>e drawn from them. Still in any 

 case the slight differences of partial pressure, 

 especially of oxygen, render the validity of any 

 explanation of the rapidity of gaseous interchange 

 within the lungs in terms of ordinary diffusion 

 extremely doubtful. A possible aid to the inter- 

 change has recently been suggested in the sudden 

 stroke of the heart, which would have an accelerat- 

 ing effect upon the liberal ion of gases from a fluid 

 under low partial pressure ; just as a tap upon the 

 sides of a glass containing soda-water wilt cause 

 bubbles of carlionic acid to be' given off. Further, 

 as already stated, some carbonic acid is combined 

 with hii'inoglohin. This combination is, like oxy- 

 luiMnoglobin, dependent upon the partial pressure 

 of the carlxmic acid, and is easily given off when 

 that pressure is lowered. Possibly the hemoglobin 

 may l>e an important carbonic acid carrier in the 

 blood. 



Effects on Respiration of the Quality am/ Qiian- 

 tity of the Gases of the Atmosphere. The respira- 

 tory mechanism, as well as the whole body, is 

 adapted to work with air of a certain composition, 

 and at a certain pressure. The mechanism can 

 adapt itself, within certain limits, to variations of 

 composition and pressure. We have to state what 

 these limits are, and what happens when they are 

 overstepped. We shall study first of all, because 

 of its practical importance, the results of breathing 

 in a confined space, or in one insufficiently venti- 

 lated. The effect upon the air of course is that the 

 proportion of oxygen is lowered, and that of car- 

 bonic acid increased. The first effect upon a per- 

 son experiencing such a state of affairs is that a 

 sense of mental and muscular fatigue occurs when 

 the proportion of carbonic acid rises to O'l per- 

 cent., the normal pro|M>rtion being 0'04 per cent. ; 

 and this is not due to the carbonic acid, out to the 

 presence of organic matter, derived probably from 

 the clothes, of the amount of which the carbonic acid 

 happens to be a measure ; for if pure carbonic acid 

 l>e introduced into the air of a room, until the pro- 

 portion rises to 1 per cent., no disagreeable sensa- 

 tions are experienced in breathing it. If the pro- 

 :rtion of oxygen lie still further diminished, or if 

 shutting t lie trachea of an animal all supply 



1"' 

 by 



of oxygen to its blood be cut off, the oxygen of 

 the blood liegins to be used up, and carbonic acid 

 begins to accumulate, and asphyxia sets in. There 

 are three stages of asphyxia. ( 1 ) The breathing 

 becomes deeper and more rapid, the blood-pressure 

 rising at the same time. (2) The respiratory 

 movements continue to increase in force and 



